Electric machines are utilized in a wide variety of applications. For example, hybrid/electric vehicles (HEVs) typically include an electric traction drive system that includes an alternating current (AC) electric motor which is driven by a power converter with a direct current (DC) power source, such as a storage battery. Motor windings of the AC electric motor can be coupled to inverter sub-modules of a power inverter module (PIM). Each inverter sub-module includes a pair of switches that switch in a complementary manner to perform a rapid switching function to convert the DC power to AC power. This AC power drives the AC electric motor, which in turn drives a shaft of HEV's drivetrain. Traditional HEVs implement a three-phase pulse width modulated (PWM) inverter module, which drives a three-phase AC machine (e.g., AC motor).
Many modern high performance AC motor drives use the principle of field oriented control (FOC) or “vector” control to control operation of the AC electric motor. In particular, vector control is often used in variable frequency drives to control the torque applied to the shaft (and thus finally the speed) of an AC electric motor by controlling the current fed to the AC electric motor. In short, stator phase currents are measured and converted into a corresponding complex space vector. This current vector is then transformed to a coordinate system rotating with the rotor of the AC electric motor.
Recently, researchers have investigated the possibility of using multi-phase machines in various applications including hybrid/electric vehicles. Higher-order multi-phase systems are characterized by additional degrees of freedom and better reliability in comparison to conventional three-phase machines, as well as by their enhanced torque producing capability.
As used herein, the term “multi-phase” refers to more than three-phases, and can be used in reference to AC electric machines that have five or more phases. One example of a multi-phase system is a five-phase system that includes a five-phase PWM inverter module that drives one or more five-phase AC machine(s). While the possibility of using five-phase systems in HEVs is being explored, a lot of work remains to be done before these inverter and motor configurations can actually be implemented particularly with respect to vector control techniques used in conjunction with such five-phase systems.
To improve dynamic performance of a multi-phase machine it is desirable to improve or increase the available mechanical torque/power that is generated and output by the multi-phase machine. One way to improve output torque (and hence machine efficiency) is to improve utilization of the inverter output voltage (also referred to as phase voltage) that is provided to the multi-phase machine.
It is well-known that addition of odd harmonics of appropriate amplitude to a fundamental wave can improve performance of a multi-phase system. For example, a well-known technique for enhancing the performance of a multi-phase machine and improving its torque producing capability and power output is commonly referred to as “third-harmonic current injection.” In third harmonic current injection, a fundamental current command and its third harmonic are used to generate voltage commands that are supplied to the multi-phase machine. Among other things, third-harmonic current injection can be used to increase the inverter output voltage and increase flux per pole of a multi-phase machine.
Accordingly, it is desirable to provide methods, systems and apparatus for controlling operation of multi-phase AC machines that are powered by a multi-phase PWM inverter module. It would also be desirable to control, optimize and/or maximize voltage utilization when using third harmonic current injection since this can help increase torque/power output from a multi-phase machine. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.